In an air drier of the type with two drying towers containing desiccant one of the towers is used for drying air emanating from the compressor in the system, while at the same time the desiccant in the other tower is regenerated by a small fraction of the dried air, which is passed back through the desiccant. The switch-over between the two towers is to be accomplished by a valve, which may be controlled to perform its function at each time the system is unloaded or in other words the pressure in the air tank to be fed by the compressor through the air drier has reached a certain level. In such a case the signal for the switch-over valve may be pneumatic.
Examples of prior designs of pneumatically controlled switch-over valves are shown in U.S. Pat. Nos. 4,331,457 and 4,398,929.
It is imperative that the switch-over valve at each and every operation reaches a position where one drying tower is sealed off and the other one connected to the conduit from the compressor irrespective of the strength of the pneumatic signal. In the opposite case, namely that the switch-over valve is stuck in an intermediate position, air from the compressor can be vented out into the atmosphere through the air drier without reaching the air tank. It goes without saying that such a situation cannot be tolerated.